This invention relates in general to an apparatus for and a method of generating and transmitting an ensemble of complex waveforms and in particular it relates to an apparatus for generating and for transmitting complex waveforms used as control signals in a time assigned speech interpolation system.
A time assigned speech interpolation or TASI system is a high speed switching and transmission system which uses the idle time (i.e., silence) in telephone calls to interpolate the speech of as many as N talkers onto approximately N/2 facilities. One such system is disclosed in the co-pending application to William A. Morgan, Ser. No. 863,902, filed Dec. 23, 1977 and assigned to the assignee of the present invention. In the TASI system disclosed therein, calls from N callers are transmitted across, for example, N/2 facility channels. Calls into the transmit side of the system are monitored by a plurality of speech detectors. When a talk burst is detected on a particular input channel by a speech detector, the talk burst is converted to digital form and that digital talk burst is briefly delayed. If a facility is available for transmission of the talk burst to the receive side of the system, the talk burst is delayed for a fixed period of time until a short control message or signalling system may be transmitted prior to the talk burst. This control message or signalling symbol is necessary so that the incoming talk burst on the receive side of the system may be identified as to its origin and directed to the proper output channel by switching means located there.
It is essential that the origin of each talk burst be unambiguously identified at the receive side of the system in order that the proper talker and listener may be interconnected. It is critical that the control signal be difficult to duplicate accidentally and, therefore, that the signal to noise ratio of each signalling symbol be as high as practical. If control signals could be accidentally duplicated, either by noise or by a speaker's voice signal, if would be possible for certain frequencies in a talker's speech train to masquerade as control signals. In this event, the talker's speech train would be momentarily directed to an improper listening party.
One arrangement for providing control signals which minimizes the likelihood of a spurious signal being confused for a control signal is provided in the present invention by a means for generating complex control signals which are the sum of, for example, four separate frequencies since a complex waveform comprised of the sum of a plurality of frequencies is not easily simulated by spurious signals.
However, in a TASI system having a relatively large number of inputs and outputs, a relatively large number of control signals are required. For example, in the aforementioned TASI system as many as 48 separate control signals may be required. If each control signal is comprised of the sum of three information bearing frequencies and one fixed frequency for timing control, then this requires that twelve separate frequencies be available for summation, in order that the 48 distinct four tone sums may be produced. It would be desirable to generate these 48 distinct symbols without the need for a large plurality of separate precise time base signal generating means.
In addition to the provision of signalling symbols in the aforementioned TASI system, signals must be transmitted between the near and far ends of the system to convey dialing information and other call status information. Such messages are composed of a series of symbols taken from the same set of twelve separate frequencies. A further need is for test signals for facility line-up and control of the gain of the far end facility interface circuit. These status and test signals are advantageously sinusoids, rather than the sums of sinusoids used for signalling symbols. It would be desirable to generate these status and test signals without need for additional time base signal generating means.
Further, since all of the aforementioned signalling, status and test signals are used on a repetitive basis, they are preferably stored in a digital memory to be recalled as needed. One method to insure high signal to noise ratio of the transmitted signalling status and test symbols would be to store the amplitudes of the signals in a digital memory with a large number of significant bits. However, since large digital memories are expensive, efforts have been made to reduce the memory capacity required in many systems. For example, it is an object of U.S. Pat. No. 3,684,680 to Amano et al to keep the memory capacity of a TASI system at a minimum.
However, in accordance with the present invention, the signal to noise ratio of complex waveforms to be transmitted is increased by statistically predicting the n equally likely amplitude ranges of the entire set of signalling symbols. Samples of the signalling symbols are stored in a digital memory in a digital code, each of the characters of which defines one of these ranges. Since there are more code values for the more likely amplitudes, these are transmitted with a higher accuracy than samples found in those having less likely amplitudes.
The prior art Bell system model .mu.255 logarithmic code converter is an analog to digital converter, which when converting analog voice signals to digital form, uses a procedure whereby a digital code is used to represent analog signals and the code values are a nonlinear function of the amplitude of the analog voice signal. In fact, in the Bell device the digital samples which are transmitted represent approximately the log of the amplitude of the sampled analog signal. In this manner, the digital samples taken of low amplitude analog signals are more accurately transmitted with respect to the actual amplitude of those signals than are samples taken of higher amplitude signals. Therefore, the percentage error of each sample is relatively constant over the entire range of analog signal amplitudes. However, one problem associated with the Bell system device is that it does not maximize the signal to noise ratio in all cases. For example, since the Bell device transmits low amplitude signals more accurately than high amplitude signals, if persons using the system consistently speak at high volume and thus at high amplitude, the signal to noise ratio of their speech signals is not maximized. It would be desirable in a TASI system to maximize the signal to noise ratio of the fixed set of signalling symbols to be employed therein.
Accordingly, it is an object of the present invention to provide an apparatus for generating a plurality of separate single frequency and multifrequency symbols which does not require a large plurality of process signal generating means.
It is a further object of the present invention to provide a means for generating a complex waveform comprising the sum of a plurality of separate frequencies.
It is a still further object of the present invention to provide an apparatus for and a method of transmitting a complex waveform which insures that the waveform may be unambiguously received.
It is a still further object of the present invention to provide an apparatus for generating a plurality of complex waveforms having a high signal to noise ratio.
It is a still further object of the present invention to provide an apparatus for generating a plurality of signalling symbols having a relatively high signal to noise ratio yet with relatively small digital storage requirements.
It is a still further object of the present invention to provide an apparatus which can generate a plurality of multi-frequency signalling symbols or test frequencies on up to 16 facilities simultaneously and independently.